Rotary blowout preventers for oil well drilling operations have existed for decades. U.S. Pat. No. 3,492,007 discloses a blowout preventer (BOP) for sealing well pressure about a rotating kelly or other production tool. U.S. Pat. No. 3,561,723 discloses a blowout preventer designed to prevent fluid from escaping from the well while the pipe string is either rotating or stationary. U.S. Pat. No. 4,098,341 discloses a rotating blowout preventer which is supplied with pressurized hydraulic fluid to lubricate and cool bearings within the BOP.
U.S. Pat. No. 4,378,849 discloses a blowout preventer with a mechanically operated relief valve to release high pressure surges in the annulus between the casing and the drill pipe sealed by the BOP packing. U.S. Pat. No. 4,383,577 discloses a rotating drilling head assembly which provides for the continuous forced circulation of oil to lubricate and cool thrust bearings within the assembly. A technique for fluidly connecting an outlet port of a BOP and an inlet of a choke manifold is disclosed in U.S. Pat. No. 4,618,314. The fluid may be injected into the blowout preventer for pressure testing and for charging the equipment with a desired fluid.
U.S. Pat. No. 5,178,215 discloses a rotary blowout preventer with a replaceable sleeve having a plurality of grippers therein. The blowout preventer disclosed in the '215 patent utilizes an inner packer which is responsive to hydraulic pressure to act against a sleeve which engages the drill pipe. The hydraulic fluid pressure which causes radial movement of the inner packer is sealed within the body of the BOP by seal assemblies which must withstand a pressure differential in excess of the difference between the well pressure and atmospheric pressure.
Improvements in rotating blowout preventers are required so that the blowout preventer may reliably withstand higher pressures, such as the high pressure commonly associated with underbalanced drilling. Underbalanced drilling occurs when the hydrostatic head of the drilling fluid is potentially lower than that of the formation being drilled. Underbalanced drilling frequently facilitates increased hydrocarbon production due to reduced formation damage, and results in both reduced loss of drilling fluids and reduced risk of differential sticking.
The disadvantages of the prior art are overcome by the present invention, and an improved blowout preventer and method of operating a blowout preventer are hereinafter disclosed. The blowout preventer is able to withstand high pressure while maintaining sealed engagement with a tubular rotating at relatively high speeds, and may also be used to seal with a non-rotating tubular.